1215 MADRONA AVE 2016-08-24 - Laserfiche WebLink

i , ' � ' ' � � ' , � �_ I � ' , , ' ' t Gustafson Residence Retaining Wall NGA File No 924115 1215 Madrona Avenue June 12, 2015 Everett, Washington Page 8 the exposed portion of the wall not exceed eight feet in height. The approximate location and cross section detail of the proposed retaining wall is sliown on the Schematic Retaining Wall Site Plan Schematic Pin Pile Wall Detail in Figure 7. However, we understand that a sewer line trends in an east to west direction through the property immediately nortl� of the proposed wall location. We recommend that the structural engineer and contractor accurately locate and confinn the location of the existing sewer prior to finalizing the design and installation of the proposed retaining wall. The retaining wall should be designed by an experienced structural engineer licensed in the State of Washington. We recommend tl�at the 3-inch square tubing ar 2-inch pin piles be galvanized extra strong (Schedule 80) steel pipe sections driven into place using a hand-held, 140-pound jackhammer or larger. The piles should be spaced at a maximum distance of four feet, and should be embedded a ininimum of 15 feet into the competent slope material. Piles that do not ineet this minimum embedment criterion should be rejected, and replacement piles should be driven after consulting with the structural engineer on the new pile locations. Our explorations encountered loose undocumented fills within the proposed retaining wall location. If large particles or debris are present within the fill, there is a possibility that this material may obstruct some piles at shallow depths. There should be contingencies in the budget and design for additional/relocated piles to replace piles that may be obstructed by debris in the fll. In addition to the miniinuin recoimnended embedment into the native inaterial, the piles should be driven to a refusal criterion of less than one inch of movement during 60 seconds of continuous driving. The wall lagging should be embedded a minimum of 12 inches into the finished slope face. Metal plates should be welded to the back of the pin piles to allow a connection for the timber lagging. We recommend that any voids bel�ind the wall be backfilled with 2-inch crushed rock up to the top of the retaining wall. For the 2-inch crushed rock, the shoring wall should be designed to resist a lateral load resulting from a fluid with a unit weight of 45 pounds per cubic foot (pc�. These loads should be applied across the pile spacing above the excavation line. The active pressure could be partially resisted by passive resistance acting on the below-grade portion of the piles. The passive resistance could be calculated based on a 120 pcf equivalent fluid density acting on two effective pile diaineters below the base of the exposed portion of the wall. This value incorporates a factor of safety of 2.0. The upper ten feet of pile embedinent should be neglected when calculating the passive resistance due to the presence of loose material on the slope. If tiebacks are utilized in flie wall design, we recommend that all later pressures be transferred to the tiebacks. A uniform surcharge of 8H should be applied to the wall design to account for seismic loading, where H is the exposed height of the wall. We should be retained to NELSON GEOTECHNICAL ASSOCIATES, INC.